Erythropoietin (“EPO”) is a glycoprotein hormone commonly associated with the maintenance of hematocrit and, more recently, tissue protection. Mature human EPO protein comprises 165 amino acids and has a molecular weight of 34 kDa, with glycosyl residues contributing about 40% of the weight of the molecule. The EPO molecule comprises four helices that interact via their hydrophobic domains to form a predominantly globular structure within an aqueous environment (Cheetham et al., 1998, Nat. Struct. Biol. 5:861-866, which is hereby incorporated by reference in its entirety). The invention derives from the discovery that certain amino acids facing the aqueous environment (i.e., away from the hydrophobic, globular central core) mediate tissue protection. Peptides can be derived or designed from an understanding of the tissue protective regions that have been identified by the Applicants.
As noted above, EPO is pluripotent. In its hormonal role, EPO regulates hematocrit through its role in the maturation of erythroid progenitor cells into erythrocytes. EPO acts as an anti-apoptotic agent during the maturation process of erythroid progenitor cells, permitting progenitor cells to mature into erythrocytes. Decreased levels of tissue oxygen (hypoxia) trigger an increased production of erythropoietin by the kidney, which results in increased erythropoiesis. Given that the kidney normally produces the majority of the serum erythropoietin, the loss of kidney function, such as in chronic renal failure, results in decreased production of EPO and often anemia. Similarly, anemia may result from other chronic conditions, such as cancer, or treatments associated with these illnesses, such as chemotherapy, which directly suppress the production of EPO. Commercially available recombinant erythropoietin has been available under the trademarks of PROCRIT, available from Ortho Biotech Inc., Raritan, N.J., and EPOGEN, available from Amgen, Inc., Thousand Oaks, Calif. and has been used to treat anemia resulting from end stage renal disease, therapy with AZT (zidovudine) in HIV-infected patients, oncology patients, and chemotherapy. Currently a hyperglycosylated erythropoietin, ARANESP™ (Amgen, Thousand Oaks, Calif.), is available for the treatment of anemia. Additionally, these compounds have been used to increase the hematocrits of patients undergoing surgery to reduce the need for allogenic blood transfusions.
Recently, several lines of evidence have suggested that EPO also functions locally in a paracrine-autocrine manner to minimize tissue damage. For example, EPO improves an hypoxic cellular microenvironment and decreases programmed cell death caused by metabolic stress. Both of these activities are moderated, in part, through EPO's interaction with a specific cell surface receptor comprised, in part, by the erythropoietin receptor (“EPOR”) protein. EPOR is an approximately 66 kDa protein and is a member of the Type-1 cytokine receptor family. This family comprises receptors that are grouped together based on the shared homology of their extracellular domains and includes receptors for interleukin IL-2, IL3, IL4, IL5, IL6, IL7, IL9, IL11, granulocyte macrophage—colony stimulating factor (GM-CSF), granulocyte colony stimulating factor (G-CSF), leukemia inhibiting factor (LIF), ciliary neurotrophic factor (CNTF), thrombopoietin, growth hormone and prolactin. The conserved extracellular domain of these receptors has a length of approximately 200 amino acids, comprises four positionally conserved cysteine residues in the amino-terminal region (Cys 294, Cys 283, Cys 248, and Cys 238, which appear to be critical to the maintenance and the structural integrity of the receptors (Murray, 1996, Harpers Biochemistry 24th ed. pp. 524-526, Appilion & Lange, Ltd.; Caravella et al., 1996, Protein: Struct. Funct. Gen. 24:394-401, each of which is hereby incorporated by reference in its entirety)), and a Trp-Ser-X-Trp-Ser (SEQ ID NO:58) motif located proximal to the transmembrane domain.
In connection with erythropoiesis, EPOR functions in a manner similar to other receptors within the Type-1 cytokine receptor family. First, the receptor ligand, e.g., EPO, binds to a preformed dimer of EPOR, (EPOR)2. It has been determined that EPO interacts with the extracellular domain of the classic (EPOR)2 homodimer receptor via two distinct regions on the ligand surface: a high affinity receptor binding site (site 1) and a low affinity receptor binding site (site 2). The amino acid sequences of EPO associated with site 1 are TKVNFY, SEQ ID NO:2, corresponding to amino acids 44-49 of SEQ ID NO:1, and SNFLRG, SEQ ID NO:3, corresponding to amino acids 146-151 of SEQ ID NO:1; the sequences associated with site 2 are VLERY, SEQ ID NO:4, corresponding to amino acids 11-15 of SEQ ID NO:1, and SGLRS, SEQ ID NO:5, corresponding to amino acids 100-104 of SEQ ID NO:1 (Cheetham et al., 1998, Nature Structural Biology 5:861-866, hereby incorporated by reference in its entirety). EPOR homodimer activation leads to tyrosine phosphorylation of signaling proteins that are associated with EPOR, e.g., Jak2 tyrosine kinases, that may in turn activate several different pathways including, for example, the phosphatidylinositol (PI) 3-kinase pathway, the Ras/MAP kinase pathway, and/or the STAT pathway. These pathways trigger the anti-apoptotic functions necessary for erythropoiesis that are mediated by erythropoietin (Kirito et al., 2002, Blood 99:102-110; Livnah et al., 1999, Science 283:987-990; Naranda et al., 2002, Endocrinology 143:2293-2302; Remy et al., 1999, Science 283:990-993; and Yoshimura et al., 1996, The Oncologist 1:337-339, each of which is hereby incorporated by reference in its entirety).
Recently, Applicants have discovered that the tissue protective properties of EPO are mediated by a receptor that comprises not only EPOR but also another receptor protein, the beta common receptor (“βc”). The EPOR/βc receptor is, in contrast to the homodimer (EPOR)2, a heterocomplex (see infra) and is known to play a role in the protection of excitable tissues. See, e.g., WO 2004/096148 and PCT no. PCT/US01/49479, filed Dec. 28, 2001, U.S. patent application Ser. No. 09/753,132, filed Dec. 29, 2000, and Ser. No. 10/188,905, filed Jul. 3, 2002, each of which is hereby incorporated by reference in its entirety. Although Applicants had established that the βc receptor is central to the tissue protective pathways in these excitable tissues, the structure of the activating ligands for the receptors was still unknown.